The present invention relates to a swim fin, and more particularly to such a swim fin which produces less resistance, and improves propelling speed when oscillated in the water.
When oscillating the legs during swimming or skin diving, the swim fins which are fastened to the feet are oscillated against the water, causing a reactive force to be produced to push to the body forwards. FIGS. 1 and 2 show a swim fin according to the prior art. This structure of swim fin 1 comprises a foot pocket 11, and a blade 12 formed integral with the foot pocket 11. The sole 111 of the foot pocket 11 and the bottom side wall 121 of the blade 12 define a 130.degree..about.150.degree. contained angle 10. Within this contained angle 10, ribs 13 are provided. When the swim fin 1 is fastened to the foot, the contained angle 101 defined between the axis longitudinally extended from the back of the user's body and the sole 111 of the foot pocket 11 exceeds 180.degree. angle. When the swim fin 1 is turned backwards in the water, a reactive force is produced and employed to the swim fin 1, causing the user to be pushed forwards. However, because the sole 111 of the foot pocket 11 and the bottom side wall 121 of the blade 12 define a 130.degree..about.150.degree. contained angle 10, the contained angle 102 defined between the axis longitudinally extended from the back of the user's body and the bottom side wall 121 of the blade 12 is smaller than 180.degree. angle, a reactive force is given from the water to the bottom side wall 121 of the blade 12 against forward movement of the body. When the user turns the foot forwards, the upper face 112 of the foot pocket 11 and the axis which is longitudinally extended from the front side of the user's body define a contained angle 103 over 180.degree. angle, and a reactive force is given to the foot to push the user forwards. At the same time, the top side wall 122 of the blade 12 which extends longitudinally from the upper face 112 of the foot pocket 11 receives the same reactive force to push the user forwards.
FIGS. 4 and 5 show another structure of swim fin 5 according to the prior art. According to this structure of swim fin 5, the top side wall 521 of the blade 52 is formed integral with the upper face 511 of the foot pocket 51, the elevation of the sole 512 of the foot pocket 51 is lower than the rear portion of the bottom side wall 522 of the blade 52, and ribs 53 are provided between the rear portion of the bottom side wall 522 of the blade 52 and the front end edge 512a of the sole 512 of the foot pocket 51. The front portion of the bottom side wall 522 of the blade 52 and the sole 512 are disposed at same elevation. As illustrated in FIG. 6, when the user turns the foot backwards, the sole 512 of the foot pocket 51 and the back side of the user's body are disposed at approximately same elevation, and a reactive force is given from the water to the sole 512 of the foot pocket 51 in direction about perpendicular to the user's body. Because the direction of the reactive force is about perpendicular to the user's body, it cannot push the user forwards. As indicated above, the contained angle which is defined between the axis longitudinally extended from the back of the user's body, the sole 111 or 512 of the foot pocket 11 or 51, the bottom side wall 121 or 522 of the blade 12 or 52, the upper face 511 or 112 of the foot pocket 51 or 11, or the top side wall 521 or 122 of the blade 52 or 12, has a great concern with the effect of the reactive force which is given from the water to the sole 512 or 111 of the foot pocket 51 or 11, the bottom side wall 522 or 121 of the blade 52 or 12, the upper face 511 or 112 of the foot pocket 51 or 11, the top side wall 521 or 122 of the blade 52 or 12.